Around 80% of the immune tissue is situated in the digestive tract. The gut lining is our first defence against the outside world – it has to deal with pathogens in everything that we ingest and must be able to prevent these pathogens entering the bloodstream.

What is the immune system?

The immune system is a huge dynamic network of cells, tissues and organs that work together to defend the body against attacks by pathogens. It is extremely complex and can recognise and remember millions of different attackers and produce cells and other compounds to kill specific identified germs.

One of the key elements of a healthy immune system is the ability to distinguish between the body’s own cells (self) and foreign cells (non-self) and to attack organisms marked as non-self.

The gastro-intestinal immune cells are known as Peyer’s patches and they protect the mucous membranes of the small intestines against infection by releasing white blood cells (T-cells and B-cells). The gut microbiota help to boost the action of these immune cells and to maintain the integrity of the gut wall.

What impacts immunity?

The immune system is affected by a wide range of external factors, including genetics, infection, stress, a diet high in sugar and alcohol, lack of sleep, smoking, lack of exercise and poor hygiene. Much of this external effect however, can be attributed to the impact of these activities on the internal gastro-intestinal tract (GIT) and gut microbiome.[1],[2],[3] The GIT is the main site of interaction between our immune system and both beneficial and pathogenic microorganisms – and the development and function of the immune system depend on these interactions.[4]The GIT is a single layer of cells, which presents a dynamic structure to recognise commensal bacteria and to limit pathogens from entering the body. The evidence suggests that the commensal flora exerts an anti-inflammatory and immune influence, providing essential nutrients, metabolising indigestible compounds, defending against pathogens and supporting the intestinal architecture. [5],[6]

How the microbiome impacts the immune system

The microbiota is established at birth when bacteria are transmitted from mother to baby and during the first year of life as the baby is exposed to different external factors. The diversity and number of bacteria in the microbiota increases over this period to drive the development of the immune system and ultimately ensure that the bacteria are recognised by the immune system as ‘self’.[7] As well as mode of birth, a number of factors have been shown to influence this development, including prematurity, hygiene, breastfeeding[8],[9] and antibiotic exposure[10]. The gut microbiota interacts with the immune system and provides signals to promote the growth of immune cells and the normal development of immune function.[11] Improper training of the immune system by the microbiota is known to result in immune-mediated diseases[12], including susceptibility to influenza, retrovirus transmission and more serious diseases such as colon cancer and auto-immune demyelination. [13]
[1] Voigt RM et al (2014) Circadian Disorganization Alters Intestinal Microbiota. PLoS ONE 9(5):e97500. Doi:10.1371/journal.pone.0097500
[2] Konturek PC et al (2011) Stress and the gut: pathophysiology, clinical consequences, diagnostic approach and treatment options. J Physiol Pharmacol 62(6):591-599
[3] Conlon MA & Bird AR (2015) The Impact of Diet and Lifestyle on Gut Microbiota and Human Health. Nutrients 7(1):17-44
[4] Macpherson AJ & Harris NL (2004) Interactions between commensal intestinal bacteria and the immune system. Nat Rev Immunol 4:478-485
[5] MacDonald TT & Monteleone G (2005) Immunity, Inflammation, and Allergy in the Gut. Science 307:1920-1925
[6] Round JL & Mazmanian SK (2009) The gut microbiome shapes intestinal immune responses during health and disease. Nat Rev Immunol 9(5):313-323
[7] Del Chierico F et al (2015) Phylogenetic and Metabolic Tracking of Gut Microbiota during Perinatal Development. PLoS ONE; DOI:10.1371/journal.pone.0137347
[8] Backhed F et al (2015) Dynamics and stabilization of the human gut microbiome during the first year of life. Cell Host Microbe 17:690-703
[9] Tannock GW et al (2013) Comparison of the compositions of the stool microbiotas of infants fed goat milk formula, cow milk-based formula, or breast milk. Appl Environ Microbiol 79:3040-8
[10] Hussey S et al (2012) Parenteral antibiotics reduce bifidobacteria colonization and diversity in neonates. Int J Microbiol doi: 10/1155/2011/130574
[11] Chow J et al (2010) Host-Bacterial Symbiosis in Health and Disease. Adv Immunol 107:243-274
[12] Petersen C & Round JL (2014) Defining dysbiosis and its influence on host immunity and disease. Cell Microbiol 16(7):1024-1033
[13] Clemente JC et al (2012) The Impact of the Gut Microbiota on Human Health: An Integrative View. Cell 148: 1258-1270

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